Hi,
Welcome to the forums.

I'm not the right person to answer your questions, but I'll pitch in just because I'm on a similar time-zone.
I really love all my gears, but I have serious hills (and only hills, no flats). I haven't ridden a SS since I was a kid. But you do mention all the correct reasons for looking into SS/FG, i.e. quite flat landscape (if I remember correctly), quite high mileage, willingness to stand up from the saddle from time to time and pray for being relived from future maintenance.
My suggestion is that you look here for a cheap used bike to have a peek into the SS/FG world, before you spend your savings on a new bike. Just remember that bike fit is important, and more so if you pedal with power. It's hard to find reliable advice on bike fit over the internet, so when you have made up your mind on which way to go, you should visit a good bike shop for advice on this.